Bacterial chemotaxis, the ability of bacteria to adapt their motion to external stimuli, has long stood as a model system for understanding transmembrane signaling, intracellular information transfer, and motility. In recent years, chemotaxis has been a paradigm for quantitative cellular studies and an inspiration for synthetic biology. Importantly, many human pathogens such Vibrio cholerae, Helicobacter pylori, and Borrelia burgdorferi (Lyme disease) rely on chemotaxis to establish infection. The sensory apparatus underlying chemotaxis, hereafter called the chemosome, displays amazing sensitivity, robustness, dynamic range and a rudimentary molecular memory. Bacteria are capable of sensing changes in chemical concentrations of less than a percent over background concentrations ranging five orders of magnitude. These properties stem from a highly cooperative excitation response and an integral feedback control that allows the system sensitivity to adapt to the surroundings. Although the molecular components of the chemosome are well characterized in the model system E. coli, we still do not understand the biochemical mechanisms responsible for function. This is because the chemosome comprises an extensive multi-component transmembrane assembly of receptors (MCPs), histidine kinases (CheA) and coupling proteins (CheW), whose detailed architecture is just emerging. The receptor arrays regulate the production of phosphorylated CheY, which directly modulates output of the flagellar motor. Given that chemosome components do not have mammalian homologs, the system is a promising target for antimicrobial agents. The current proposal continues efforts to understand how the chemosome is assembled from its components, how signals transverse the membrane, and ultimately, how chemoreceptors regulate CheA activity by restructuring the receptor arrays. To solve these problems studies will be undertaken on isolated chemosome components, reconstituted complexes, and native receptor arrays in whole cells through a combination of biophysical and biochemical techniques that address these systems over a span of resolutions and length scales. These methods include X-ray crystallographic structure determination of binary and ternary complexes, pulsed dipolar ESR spectroscopy (PDS) of larger assemblies and membrane incorporated receptors, electron microscopy of 2D crystals and electron cryotomography of native receptor arrays. Soluble, chemoreceptor maquettes have been designed to mimic receptor oligomeric states necessary for kinase activation. Full-length transmembrane receptors will be studied in the context of nanodisks, where they are fully functional. Of additional interest is a soluble aerotaxis receptor Aer2, which is a tractable subjet for probing interdomain communication and the ubiquitous HAMP signal-transducing module. Structural studies will be coupled to cellular activity through in vivo activity assays. Through ths work understanding will be deepened in how receptors produce kinase-on and -off states and the function of high-order assemblies in generating the hallmark high gain, sensitivity and dynamic range of the chemosome.